1. Field of Invention
The present invention relates to an electrostatic capacitance sensing device that reads the surface contours of a fingerprint or other target object having extremely small ridges and valleys, by sensing electrostatic capacitance, which changes according to the distance from the surface of the target object.
2. Description of Related Art
Japanese Unexamined Patent Publications No. 11-118415, No. 2000-346608, No. 2001-56204, No. 2001-133213 disclose that related art electrostatic capacitance sensing devices, that are used in fingerprint sensors and so forth, a sensor electrode and a dielectric deposited on the sensor electrode, can be formed on a single-crystal silicon wafer. The principle of operation of a related art electrostatic capacitance sensing device is shown in FIG. 1. One of a pair of electrodes in a capacitor is a sensor electrode, and the other is grounded to a human. Furthermore, a dielectric is formed on the uppermost surface of the electrostatic capacitance sensing device. The capacitance CF of this capacitor changes according to the ridges and valleys in a fingerprint contacting the surface of the dielectric. Meanwhile, the semiconductor substrate is equipped with a capacitor that forms electrostatic capacitance Cs. These two capacitors are coupled in series, and a predetermined voltage is applied. The application of a voltage causes a charge Q corresponding to the ridges and valleys in a fingerprint to be formed between the two capacitors. This charge is sensed using ordinary semiconductor technology and the surface contours of the target object are read.